132 MAZATLAN BIVALVES 
Hab.—Mazatlan ; 1 sp. in Spondylus; L’pool Col. 
Tablet 607 contains a sketch. 
Faminty ARCADZA. 
Genus ARCA, Linn. 
Area, pars, Linn. Lam. Arca, Swains. 1840.—Senilia, Gray, 
1848. 
The Arce appear to divide themselves into two natural 
groups, those which live freely in sand or mud, like cockles, 
whose name they commonly bear; and those which live nest- 
ling in crevices or affixed to rocks by a horny byssoid pedal 
appendage. The former have stout, regular, strongly ribbed 
shells, and are considered by Swainson the typical species ; 
the latter have irregular shells, generally with a thin or shaggy 
epidermis, and a more or less developed ventral gape. These, 
which are the typical species of most authors, form Swainson’s 
genus Byssoarca. 
180. Arca GRANDIS, Brod. & Sow. 
Zool. Journ. vol. iv. p. 865.—Rve. Conch. Ic. pl. 1, f. 4.—Hanl. 
Deser. Cat. p. 160.—B. M. Cat. D’ Orb. Moll. p. 82, no. 729.— 
C. B. Ad. Pan. Shells, p. 259, no. 417. 
This species, the W. Pacific analogue of the East Indian 
A. senilis, is known from it by the greater number of ribs, 
(25-30) and by the teeth which are comparatively narrow. 
They are however wider than in A. tuberculosa. The shell 
in its young state is generally subequilateral, subquadrate, 
often slightly inzequivalve, with the epidermis for the most part 
smooth and persistent, displaying here and there hairs, and 
cancellating scales across the grooves. The ribs are then often 
eranulose. As it advances to maturity, the posterior part 
generally becomes much produced, till the aspect of the shell 
becomes very inecuilateral, and not unlike that of A. tuber- 
culosa. It may however always be distinguished from it by its 
greater solidity, the smaller number of ribs, and by the grooves 
_ which nearly equal the ribs in breadth. The epidermis grad- 
ually becomes thick and rough, forming in concentric layers, 
which are very conspicuous across the grooves. The umbonal 
portion generally becomes decorticated. The ligament is 
coarse and solid, filling up the whole of the hinge area except a 
